1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine and a printer.
2. Description of the Related Art
In the image forming apparatus such as the printer, the copying machine and a facsimile that have hitherto employed an electrophotographic technology, a fixing apparatus 50 having a configuration illustrated in FIG. 14 has been widely adopted as means of fixing an unfixed toner image formed and borne on a recording material. The fixing apparatus 50 includes a heating unit 110 coming into contact with an unfixed toner image. The heating unit 110 has a rotary fixing member (hereinafter simply referred to as “sleeve”) 102 such as an endless belt and a roller. The fixing apparatus 50 has a pressure roller 104 defined as a nip forming rotary member. A recording material P is conveyed and pinched by a fixing nip portion N formed between the sleeve 102 and the pressure roller 104, and undergoes image-fixation therein.
The image forming apparatus including the fixing apparatus 50 having the configuration described above has a problem that a toner T adhered to the surface of the sleeve 102 and the surface of the pressure roller 104 becomes offset to the recording material P.
Proposed for preventing this phenomenon is a method of applying a bias to a rotary fixing member or a nip forming rotary member of the pressure roller etc as hitherto typified by fixing apparatuses discussed in Japanese Patent No. 3053459, Japanese Patent Application Laid-Open No. 2000-338807 and Japanese Patent Application Laid-Open No. 2003-287967.
According to Japanese Patent No. 3053459, the bias is applied to only a fixing roller, and the offset is thus prevented. According to Japanese Patent Application Laid-Open No. 2000-338807 and Japanese Patent Application Laid-Open No. 2003-287967 each propose the method of applying the bias to both of a fixing film and the pressure roller.
As in Japanese Patent Application Laid-Open No. S64-065589, there is proposed a method of controlling a surface potential by coating a fluorine surface active agent over the surface of the roller.
The prior arts given above sufficiently satisfied the then-desired performance. In recent years, however, speed-up of the image forming apparatus has been requested by users, and a further improvement of the performance in terms of a task of flexibility to multifarious types of sheets has been also demanded of the image forming apparatus.
As a result, according to the configuration shown in Japanese Patent No. 3053459, it is difficult for one single bias power supply to set an optimum bias to the multifarious types of sheets and a variety of environments. Therefore, this has proven that there is a case in which the offset can not be completely restrained with respect to a specified type of sheet.
The system shown in Japanese Patent Application Laid-Open No. 2000-338807 solves the problem given above but needs to have two power supplies, which leads to a rise in costs. In terms of a trend toward lower prices over the recent years, a cost reduction is an indispensable issue, and the user's requests can not be satisfied by any technologies having none of high performance at low costs.
Japanese Patent Application Laid-Open No. 2003-287967 proposes a method by which one single power supply applies the bias to both of the fixing roller and the pressure roller. This method, however, has also proven that there is the case in which the offset can not be completely restrained with respect to the specified type of sheet as in the case of Japanese Patent No. 3053459.
As to Japanese Patent Application Laid-Open No. S64-065589, it is difficult to set the optimum surface potential to the multifarious types of sheets and the variety of environments, and it has been recognized that there is the case in which the offset can not be completely restrained with respect to the specified type of sheet.